Method of Making Dental Restorations from Sintered Preforms

ABSTRACT

A method is provided for shaping a custom dental restoration from a preform, wherein the preform comprises a preform body and a preform stem. A method is further disclosed for generating one or more nesting positions for the restoration design within the geometry of the preform body relative to the position of the preform stem. A method is further disclosed for generating machining instructions based on the selected nesting position to optimize machining for chair-side applications.

RELATED APPLICATIONS

This application is a continuation U.S. patent application Ser. No.16/426,180, filed May 30, 2019, now U.S. Pat. No. 11,266,485, which is acontinuation of U.S. patent application Ser. No. 16/291,276, filed Mar.4, 2019, now U.S. Pat. No. 10,973,616, which is a continuation of U.S.patent application Ser. No. 15/259,550, filed Sep. 8, 2016, now U.S.Pat. No. 10,258,440, which claims the benefit of and priority to U.S.Provisional Patent Application No. 62/215,525, filed Sep. 8, 2015, allof which applications are incorporated herein by reference in theirentireties.

BACKGROUND OF THE INVENTION

Ceramic materials known for use in the field of dentistry provide highstrength restorations such as crowns, bridges, and the like. Someceramic materials have flexural strength values exceeding 800 MPa whenfully sintered, resulting in restorations that are resistant tochipping, breakage and wear. Material advances provide enhancedaesthetics in color and translucency while maintaining acceptablestrength, and restorations may be manufactured from these materials in acost effective manner.

Dental restorations created by computer assisted design processes may bemilled by CAM processes from porous ceramic materials in the green orbisque ceramic stage, using an enlargement factor to accommodatereduction in overall size upon heating to full density. After milling,the porous restoration design is sintered to full density to produce afinal restoration. Disadvantageously, the separate steps of milling theporous ceramic dental design and sintering the milled shape to form thefinal dental restoration, may preclude dentists from making chair-sideceramic restorations, increasing the amount of time a patient must waitfor repair.

To reduce the amount of material waste to make a restoration,US2006/0204932 discloses an assemblage or library of “smart” mill blankspre-configured into geometries and sizes that closely resemble the finaldental parts. Material waste may be reduced compared to traditional millblanks that have a single size and shape, which is desirable when usingprecious or semi-precious materials. The smart mill blank library isdescribed as comprising a series of blanks with geometries that differother than by scale, and preferably having at most, one symmetric plane.The blank is mounted in a shaping apparatus by a milling holder that hasan orientation-specific attachment key for the milling machine.

Methods of making ceramic restorations from near net shape millableblanks are also known, for example, from commonly owned U.S. Patent Pub.No. 2013/0316305, which is hereby incorporated by reference in itsentirety. In this document, a kit is disclosed containing millableblanks of various shapes, each shape designed to closely replicate arestoration shape thus minimizing material removal in chair-sideprocesses. The kit comprises a variety of shapes and shades ofrestoration blanks, as well as chair-side software, and a chair-sidemilling machine to convert millable blanks into finished, contouredrestorations by a dentist.

SUMMARY OF THE INVENTION

A method for making a custom dental restoration, such as a crown, from amachinable preform is disclosed. The methods are suitable for shapingmaterials that have sufficient strength and hardness properties intodental restorations that may be directly inserted into the mouth of apatient without the need for a further processing step to strengthen thematerial after it has been shaped. Methods and apparatus describedherein reduce the time required to prepare a finished dentalrestoration. Advantageously, fully sintered materials known for strengthand durability, such as sintered zirconia, may be shaped directly intorestorations in chair-side applications or in a laboratory withoutrequiring post-shaping sintering processes. A novel sintered, shapedpreform and shaping tool are described, as well as unique nestingmethods, machining strategies, and tool paths.

A sintered preform from which a final, custom dental restoration isshaped comprises a body of sintered material and a stem projecting fromthe center of the preform body. When used in a dentist office chair-sidemilling machine, the time to create a custom finished product issignificantly reduced. Unique features of the sintered preform includethe size and shape of the preform body which accommodate most customrestoration designs, and reduce the amount of sintered material to beremoved during the process of shaping a dental restoration.Advantageously, the shape of the preform accommodates multiple optionsfor nesting the dental restoration, and methods described herein forselecting nesting positions based on stem placement options enable thegeneration of unique tool paths for shaping dental restorations fromsintered materials.

A method for making the sintered preform is also disclosed thatcomprises the steps of obtaining unsintered material, shaping theunsintered material into an unsintered, intermediate shaped form havinga body and a stem, and sintering the intermediate shaped form to fulldensity to form the sintered preform. Unsintered material may beobtained in the form of a block and then milled into a unitary shapedform that comprises the body (201), stem (202) and optionally, anattachment (203), that is enlarged where necessary to accommodateshrinkage upon sintering. Alternatively, unsintered material may bemolded into the unsintered intermediate shaped form, for example, byinjection molding. In a further alternative, unsintered material may befirst molded into a first shaped form, and then, subsequently milled forshape refinement into a second shaped form prior to sintering.

A method for making a custom dental restoration comprises designing acustom restoration by a known CAD (computer-aided design) process,nesting a CAD dental restoration design within a computer model of apreform body, generating tool paths from a machining strategy and thepositional information of the nested restoration design, and machiningthe sintered preform into the final restoration. In one embodiment, amethod comprises nesting the restoration design within the preform body,wherein the preform stem is positioned outside of the proximal contactareas of adjacent teeth. A method further comprises generating at leasttwo tool paths for shaping the occlusal side of the restoration or innersurface of a restoration, wherein a first tool path has a tool pathentry point adjacent the preform stem, and a second other tool path hasa tool entry point on the front side of the preform—the side that isopposite the stem. In another embodiment, at least two tool paths areprovided for shaping the restoration from the occlusal side and at leasttwo additional tool paths are provided for shaping the inner surfaceside of the restoration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A. A graphic representation of a side perspective view of a dentalrestoration shaped from a preform and attached to a stem according toone embodiment.

FIG. 1B. A graphic representation of a dental restoration according toone embodiment.

FIG. 1C. A graphic representation of a computer model of a dentalrestoration nested within a model of the preform.

FIG. 2A. A top perspective view of a graphic representation of a preformon a mandrel according to one embodiment.

FIG. 2B. A bottom perspective view of a graphic representation of apreform according to one embodiment.

FIG. 2C. A bottom view of a graphic representation of a preformaccording to one embodiment.

FIG. 2D. A side view of a graphic representation of a preform accordingto one embodiment.

FIG. 3. A side view of a graphic representation of a grinding toolaccording to one embodiment.

FIG. 4A. A graphic representation of a tool path applied to arestoration design.

FIG. 4B. A graphic representation of a portion of a tool path on a slopeaccording to one embodiment.

FIG. 5A. A graphic representation of a down-hill tool path according toone embodiment.

FIG. 5B. A graphic representation of an up-hill tool path according toone embodiment.

FIG. 6A. A graphic representation of a nesting method showing contactareas according to one embodiment.

FIG. 6B. A graphic representation of a nesting method showing contactareas between adjacent teeth of a preparation from an occlusal view.

FIG. 7A. A graphic representation of an occlusal view of a restorationdesign and nesting method according to one embodiment.

FIG. 7B. A graphic representation of an inner-surface view of arestoration design and nesting method according to one embodiment.

FIGS. 8A-8G. A graphic representation of one embodiment of a machiningstrategy for making a restoration from a sintered preform.

FIG. 9. A flowchart of a method for nesting a restoration design in apreform according to one embodiment.

FIG. 10. A flowchart of a method for nesting a restoration design in apreform according to one embodiment.

DETAILED DESCRIPTION

A method for making a custom dental restoration is described herein. Adental restoration machined from a machinable preform is illustrated inFIG. 1A (100). In one embodiment, the preform comprises a sinteredpreform material that may be machined chair-side in a dentist's officeinto a final dental restoration, such as a crown, and secured directlyinto the mouth of a patient without requiring post-process sintering. Amethod is provided for machining the sintered preform into a customfinal dental restoration that reduces the time required to prepare afully sintered final dental restoration. The methods and apparatusdisclosed herein comprise novel features including a unique preformdesign, nesting strategies, tool paths and machining strategies. In afurther embodiment, a kit is provided that comprises a preform, agrinding tool, and computer programs or modules for nesting arestoration design within the preform and generating tool to shape afinal restoration chair-side, without the need for sintering aftershaping the restoration.

Illustrated in FIGS. 2A and 2B, one embodiment of a preform (200)comprises a body (201) from which a dental restoration (101) ismachined, and a stem (202) that projects from the body (201). Asillustrated in FIG. 2A, the sintered preform (200) may comprise anattachment (203) that is attached to a mandrel (216), for securing thepreform (200) to a shaping machine. Machines suitable for use hereinhave at least 3 axes, such as a 3+1 axis CNC machine. After machining,the restoration (101) is complete, the remaining stem (102) of thesintered preform may be easily removed for immediate placement on apatient's tooth preparation.

In one embodiment, a sintered preform (200), illustrated in FIGS. 2A,2B, 2C, and 2D, has a circular-cylindrical body (201) having a curvedouter surface (204) and a cylinder length (line A-A′) that is in thez-axis direction of a shaping tool of a CNC machine. A center portion(205) of the body (201) extends between bottom end region (206) and atop end region (207). In FIGS. 2A, 2B, 2C, and 2D, the length (A-A′) ofthe cylindrical body (201) is substantially orthogonal to the length(along line C-C′) of the stem (202) in the y-axis direction. In thisembodiment, the stem projects from a stem contact point on the outer,curved surface (204) of the center portion (205) of the cylindricalbody, and extends to an attaching member (203) for direct or indirectattachment to a shaping machine. The sintered preform of FIGS. 2A-2Dfurther comprises a cavity (208). The curved outer surface (204) of thecenter portion of the cylindrical body exemplified in FIGS. 2A-2C issubstantially smooth.

In the embodiment illustrated in FIG. 2C, the cylindrical body (201)comprises a substantially circular cross-section (line B-B′) parallel tothe bottom and top end surfaces (209 and 210). The ends of thecylindrical body comprise circular bottom and top end surfaces (209 and210, respectively). In this embodiment, the bottom end surface (209) hasa concavity (211) from which a cavity (208) extends inwardly into thecylindrical body and defines a preform inner surface (212). The outerdiameter of a circular cross-section of the center portion from whichthe restoration design is shaped may be from about 12 mm to about 20 mm,or from about 13 mm to about 18 mm, or from about 14 mm to about 17 mm.The length of the preform body between the top end and the bottom end issufficient to accommodate the height of most dental restoration designswhen measured, for example, from the highest point of the occlusalsurface to the lowest point on a tooth margin; thus, the length of thepreform body or the center portion of the preform body may be less than20 mm, or less than 18 mm, or less than 16 mm, or less than 15 mm, ormay be between about 10 mm and 15 mm. In some embodiments, the ratio ofthe cross-sectional diameter of the center portion to the length of thepreform body is greater than 1.0:1.0.

The preform body (201) may comprise a shape other than a cylinder, forexample, an ellipsoid cylinder, a polyhedron, curved polyhedron, acylinder with flattened surfaces, a square, a square with rounded edges,and the like. In one embodiment, the dimensions of a preform body havingan irregular shape provide for a full rotation of the restoration designaround the z-axis. A preform body may have a cross-sectional geometry(parallel with top and bottom surfaces) with an inscribed circlediameter greater than approximately 12 mm and a circumscribed circlediameter less than approximately 20 mm at the stem contact point.

In some embodiments, the body (201) may have a substantially uniformcross-sectional dimension throughout the body length, wherein the topend surface (210) and/or bottom end surface (209) are flat.Alternatively, the preform may comprise a shaped edge (213) between thecurved outer surface (204) of the preform body (201) and top end surface(210) and/or bottom end surface (209), defining the top and bottom endregions (207 and 206, respectively). The preform body may also, oralternatively, have a shaped edge (213′) around the cavity (208)perimeter. For example, as illustrated in FIGS. 2A-2D, a filleted edgesurrounds the bottom end surface concavity (211).

In some embodiments, the preform comprises at least one chamfered edge,at least one filleted edge, or both a chamfered edge and filleted edge.For example, as illustrated in FIGS. 2A-2D, a filleted edge surroundsthe bottom end surface concavity (211). In other embodiments, the shapededge (213) between the cylindrical curved outer surface (204) and bottomor top end surfaces (209, 210) may be chamfered. The shaped edge (213′)around the cavity (208) perimeter may be chamfered. The bottom and/ortop end regions (206 and/or 207) of the preform body may have across-sectional width or diameter that is smaller than the width ordiameter of the center portion (205). In some embodiments, a preformhaving a shaped edge has less sintered material to be removed whenmaking the final restoration crown shortening the shaping time. Theshaped edge may also facilitate access to the cavity by a shaping tool.

The stem (202) supports the sintered preform body (201) in the shapingmachine while shaping the body into a final dental restoration. The stem(202) bridges the cylindrical body (201) and optional attaching member(203), and the stem length axis (C-C′) is orthogonal to the length ofthe cylindrical body (A-A′). In some embodiments, the stem length axisis within about 30 degrees or within about 45 degrees of orthogonal,relative to the body. In one example, the first stem end extends fromthe center portion of a cylindrical body, and in a further embodiment,the stem contact point is approximately equidistant between the bottomend (209) and the top end (210) of the preform body.

In some embodiments, prior to shaping the sintered preform, a stemlength (along line C-C′) may be greater than the width of the stem atthe first stem end (214) proximate the cylindrical body. (For purposesherein, width may be used interchangeably with diameter, for example, inembodiments in which the stem has a circular cross-section). The stemlength may be between about 3 mm and about 12 mm, or between about 3 mmand about 10 mm. In some embodiments, the stem length may be greaterthan about 3 mm, or greater than about 4 mm, or greater than about 5 mm,or greater than about 6 mm, or greater than about 8 mm. In oneembodiment, the width (diameter) of the first stem end (214) is lessthan the width (diameter) of the second stem end (215) proximate theattaching member (203). The width (diameter) of the first stem end maybe about 1 mm to about to about 4 mm, or from 1 mm to about 3 mm, orabout 1.5 mm to about 3 mm, or 1.5 to about 2.5, or less than about 4mm, or less than about 3 mm, or less than about 2.5 mm. In someembodiments, the ratio of stem length to the first stem end (214) widthor diameter (proximate the preform body) is greater than about 1.5:1, orgreater than about 2:1, or greater than about 3:1.

In one embodiment, the stem length is greater than the diameter of theshaping tool. In this embodiment, the stem length forms a space betweenthe preform body (201) and an attachment (203) for entry of a shapingtool near the first stem end for entry to a first tool path without thetool tip contacting the sintered preform material, and thereby reducingtool wear. The flex strength of the stem (202) at the first stem end(214) is sufficiently high to support the sintered preform (200) duringmachining from a sintered state, and sufficiently low for the finishedrestoration to easily be broken off from the stem, for example, by hand.The stem shape may be a cylinder, tapered cylinder, cone, prism or thelike, and is connected to the center portion of the preform body at thestem contact point by a first stem end.

As exemplified in FIGS. 2B-2D, a preform cavity (208) begins at an endsurface (210, 209) of a bottom or top end region (206 or 207) andextends into the body (201). The contour of the cavity forms a preforminner surface (212) that is accessible by a shaping tool. The shape ofeach cavity may be the same or different, and may comprise, but is notlimited to, an inverted cone, dome, cylinder, trough, or the like, ormay have an irregular shape. An opening or breakout geometry of thecavity may have a width (or diameter, for example where the breakoutarea is circular) that is between about 20% and 80% of the outerdiameter or width of the center portion of the preform body. The cavityopening or break-out dimension may have a surface area that is about 50%to about 80% of the surface area of a top end face, a bottom end face,or a center portion cross-section. The approximate cavity depth may bebetween 5% and 50% of the length of the preform body. A circular cavityopening may have an inner diameter of up to about 75 percent of theouter surface diameter of the preform body when measured from the endface.

The sintered preform may be attached directly or indirectly to a shapingmachine by an attaching member (203) that is joined to the second end ofthe stem (215). FIG. 2A illustrates an embodiment in which the attachingmember (203) is connected to a mandrel (216) for indirect attachment ofthe preform to a shaping machine. The attaching member (203) may have asubstantially flat surface (217) shaped as a rectangle, circle, orsquare, as exemplified in FIG. 2A, or, for adhesive attachment to amandrel.

In further embodiments, methods are provided for machining a dentalrestoration from a sintered preform from CAD/CAM-based systems. Dataregarding anatomical information about the patient's tooth preparation,and optionally, surrounding teeth and original tooth structure, arecollected and stored in a computer or computer storage media. A computermodel of a desired restoration design (107) may be created manually byan operator, or automatically proposed, in a dental restoration CADsystem. Known design systems, such as IOS FASTDESIGN™ System (IOSTechnologies, San Diego, Calif.), are suitable for designing dentalrestorations for use herein. The computer model of a patient's dentalrestoration design may be selectively nested within a computer model ofthe preform (108) to establish optimal machining conditions for aselected CNC machine and shaping tool, such as a milling or grindingtool. A suitable chair-side milling machine includes, but is not limitedto the TS150™ chair-side milling system (IOS Technologies, San Diego,Calif.), and an exemplary grinding tool (300) suitable for use herein isillustrated in FIG. 3. Positional data of the nested restoration design(for example, FIGS. 6A, 6B, 7A and 7B) may be provided to the CAM systemto calculate tool paths from machining strategies (FIGS. 4A, 4B, 5A and5B) including lace direction, XY step over, maximum Z increments, feedrates, coolant parameters, and the like, based on the milling machineselected and properties of the grinding tool, as further describedherein.

A computer-implemented method for nesting a restoration design withinthe geometry of a preform is provided. In one embodiment, dental CADsoftware is used to generate a computerized 3D model of the restorationdesign (107). The 3D model of the dental restoration design and acomputer model of a preform (108) are aligned generally along z-axeswithin a CAD system. A restoration design may be nested within thegeometry of a preform so that the cavity on the inner surface (105) ofthe restoration design (e.g., the inner surface of the dentalrestoration that is designed to contact and attach to a toothpreparation), is aligned to be adjacent the cavity (109) of the preform.By aligning the restoration's concave inner surface (105) adjacent thecavity of the preform, the amount of material removal during the shapingprocess is reduced.

In one embodiment, the step of nesting includes rotation around thez-axis, and translation in the z-, x- and y-axes, orienting the positionof the preform stem relative to the computer model of the restorationdesign. The position of the stem relative to the outer surface of therestoration design may be selected that optimizes machining conditionsand material removal. In one embodiment, a restoration design is nestedwithin the geometry of a preform so that the first stem end is locatedon or near the parting line of the restoration design, between occlusalsurface (103) and margin (104), as exemplified in FIGS. 1A and 1C. Therestoration design may be nested so that a minimum distance value isestablished between the first stem end and the restoration tooth margin(104) and/or the proximal contact points. By nesting the restorationdesign so that the preform stem contact point is at a distance from themargin or mesial and distal contact points of the restoration,variations in the surface geometry of the final dental restoration thatmay result upon removal of the stem may be minimized in these areasthereby increasing the likelihood that optimal fitment is achieved inthe final restoration.

FIGS. 6A and 6B illustrate a method (600) for nesting the restorationdesign (601) within the preform geometry comprising arranging computermodels of the restoration design and preform at a rotation around thez-axis, and positioning the length of the stem (along the y-axis (C-C′))and stem contact point outside of the proximate contact areas (606and/or 607) of neighboring teeth. A proximal contact area (606 or 607)refers to the tooth surface that may touch an adjacent tooth in the samearch (FIG. 6B) when the final restoration tooth (601) is placed on thepatient's tooth preparation. A distal proximal contact area (606)includes the distal surface area of the restoration tooth that maycontact the mesial surface a more posteriorly positioned adjacent tooth(612); the mesial proximal contact area (607) encompasses the mesialsurface of the restoration tooth that may contact the distal surface ofan anteriorly oriented adjacent tooth (613). Distal and mesial proximalcontact areas (606 and 607) may be identified by a technician, orautomatically identified by the restoration design software. By nestingthe restoration design within the preform so that the stem contact pointis outside of the mesial and/or distal contact areas (606, 607), thestem will not be coincident to a mesial or distal contact point in thefinal restoration.

The preform stem may be optionally positioned on the buccal surface(610) or on the lingual surface (611) of the restoration design (601),outside of the proximal contact areas (606 and 607). In a furtherembodiment, the model is rotated for alignment of the axis of the stemlength (C-C′) between a buccal-lingual plane and a mesial-distal planeof the restoration design. A restoration design may be nested within amodel of the preform body to achieve a mesio-buccal stem placementposition (603), between the buccal surface and the proximal mesialcontact area, or alternatively, a mesio-lingual stem position (605)between the lingual surface and the mesial proximal contact area of thedesign. In further alternative embodiments, a disto-buccal stem position(602) may be established by orienting the stem between the distalcontact area and the buccal surface, or a disto-lingual stem position(604) between the distal contact area and the lingual surface,respectively.

The restoration design may also be translated along one or more of x-,y- and z-axes, when nesting within the geometry of the preform model.For example, translation along the z-axis adjusts the distance betweenthe occlusal surface (103) of a restoration design and top end (210) ofthe preform model, or the distance between the margin (104) of arestoration design and bottom end (209) of the preform. Translation ofthe restoration design along the z-axis may be performed to locate thefirst stem end at a distance between the occlusal surface (103) andmargin (104), away from the margin of the restoration design. Moreover,the restoration design may be translated along the z-axis, y-axis orx-axis to align the cavity (105) of a restoration design adjacent apreform cavity (208) to facilitate access of the machine tool (300)directly into the preform cavity to shape the restoration design's innersurface. The restoration design may be translated along the y-axis orx-axis to change the restoration design's position relative to the outersurface of the preform body. For example, the restoration design may bepositioned directly adjacent the first stem end minimizing the amount ofmaterial (106) to be removed from the dental restoration outer surfaceafter shaping. In an embodiment, threshold parameters may be establishedthat provide for a minimum distance between the outer surface of thepreform body and the outer surface of the restoration design.

Positional data of the nested restoration design may be provided to theCAM system to calculate tool paths to shape the final restoration fromthe preform. A method is provided comprising obtaining a machiningstrategy that comprises two or more machining steps to shape arestoration from the nested design. Each machining step may comprise atool path for machining a portion of the restoration including machiningstrategy elements such as lace direction, XY step over value, maximum Zincrements, feed rates, coolant parameters, and the like. In oneembodiment, a tool path is provided overlaying a restoration design thatfollows a lacing, or zig-zag, pattern (400) as illustrated in FIGS. 4Aand 4B. The parallel lacing pattern comprises generally straightsequential tool path lines (405) separated by a distance (401). Toolpaths may be established using linear interpolation methods based on XYZmachining positions, and appropriate spacing between tool path lines(405) or passes, to optimize machining conditions. The planar spacingbetween lines of the lacing pattern, or step-over distance (401), may bean arbitrary increment, for example, based on tool dimensions (407).Alternatively, the step-over distance (401) in the Y-direction betweensequential lines (405) of the tool path may be independently adjusted toinsert additional tool path lines (405). For example, if the distance(406) between two tool path lines (405′) exceeds a threshold value inthe Z direction (e.g., a maximum Z increment value in a Z-negativedirection) in one area of the tool path sequence, the step-over distance(401) between adjacent lines may be decreased and additional tool pathlines may be inserted in that area to decrease the Z increment distancebetween two tool path lines until the threshold value is met or notexceeded.

FIGS. 5A and 5B depict cross-sectional representations of FIG. 4B (404),and the movement of a tool (e.g., 300 407, 505) across a slope (500).The Z-axis of the grinding tool (300) moves toward Z-positive (402), forexample, as the tool is lifted away from the surface to be machined, andprojections through the tool path may occur towards Z-negative (403).Where the step-over (503) position of the tool (505) is towardZ-negative, a ‘down-hill’ (FIG. 5A) grinding path (501) results. Atleast a portion of the material removal (507) in a down-hill movementoccurs by the tool tip (302, 506). Material removal by the tool tip mayresult in heating of the tool (300), wear on the tool tip (302), andexcessive wear on the grinding media (303), such as diamonds embedded inan alloy coating on the tool shank (301). Where the step-over (504)position is inclined toward Z-positive (FIG. 5B), an ‘up-hill’ (502)grinding path provides material removal (507) by a tool side surface(508) reducing material removal by a tool tip (506), thereby reducingwear on the tool. A method is provided comprising machining strategiesthat are optimized for material removal in up-hill movements. However,where z-negative milling is unavoidable, a method is provided thatlimits the amount of continuous machining that occurs in the (down-hill)z-negative direction, for example, by minimizing the length of each toolpath.

In one embodiment, a method for minimizing the percentage of Z-negativedirection movement of a grinding tool comprises implementing a nestingstrategy for optimizing the nesting position of a restoration designwithin the preform geometry, and generating tool paths from the selectednesting position. In one method, stress or wear on a grinding tool isreduced by selecting a nesting position that reduces the length of toolpath lines across a dental restoration design. For some dentalrestorations, the longest dimension of the restoration design is thewidth between buccal (704) and lingual (705) surfaces (represented bye.g., line 703), and/or the width between mesial (706) and distal (707)surfaces. Machining strategies in which a tool passes either parallel tothe longest dimension of a tooth, or orthogonal to the longestdimensions of a tooth, may place stress on the grinding tool. In oneembodiment, a method comprises selecting a nesting configuration inwhich line (703) separating the buccal and lingual sides of therestoration design (700), is not orthogonal to the stem length, so thattool path lines that run orthogonal to the stem length (Y-axis) are notparallel to line (703). In one embodiment, an embodiment of the methodis provided for nesting a restoration design (depicted from the innersurface (FIG. 7B, 708) and occlusal surface (FIG. 7A, 709)). Therestoration design, rotated around the Z-axis, is positioned so thatline (703) separating the buccal and lingual surfaces is off-set fromthe stem length (y-axis) (701), and line 703 is not parallel with thestem length axis. In another embodiment, the method comprises nesting arestoration design within a preform body so that a line (703) separatingthe buccal and lingual surfaces is both off-set from the stem lengthaxis (y-axis) and off-set from orthogonal to the stem-length axis, sothat the line (703) is neither parallel to nor orthogonal to thestem-length axis.

For illustrative purposes, a quadrilateral box (702) is depicted havingadjacent buccal, lingual, mesial and distal side surfaces bounding theouter surfaces of restoration design. In this embodiment, therestoration design within the box is angled in a diamond configuration(710) relative to the position of the stem (701), so that the sides areneither orthogonal nor parallel to the stem length. Thus, a method isprovided comprising nesting a restoration design in diamondconfiguration relative to the stem and providing a tool path sequencehaving tool path lines orthogonal to the stem length (y-axis) (701), andwhen used, e.g., with a parallel lacing tool path pattern (with XYZmachining positions), the length of the resulting tool paths through themesial-distal width of the tooth is reduced. In one embodiment, theY-axis of the stem of the preform design is positioned at an anglebetween mesial and distal (608) surfaces and buccal and lingual (609)surfaces, resulting in a diamond configuration as seen in FIGS. 7A and7B, reducing the length of the tool path across a restoration whengrinding in a down-hill (Z-negative) position for at least a portion ofthe grinding sequence.

In one embodiment, a restoration design is nested in a plurality ofpositions, as described above. In one embodiment, for each nestingposition, a negative slope value is calculated for the inner surface orocclusal surface. Negative slope values are calculated as the percentageof a restoration design surface area that is determined to have anegative slope greater than a threshold angle, such as 10°, 15°, 20°, or30°, or greater, when viewed from the machining direction. Optionally, anegative slope value for a restoration may be the sum of the percentageof surface area having a negative slope greater than a threshold anglefor both occlusal and inner surfaces. The surface area with negativeslope corresponds to area in which shaping may be performed by down-hill(Z-negative) machining of the tool at an angle greater than or equal tothe threshold angle. For example, in one embodiment using an stl. fileformat, an occlusal or inner surface of a restoration design may beanalyzed to determine what percentage of a triangulated surface geometryis sloped greater than a threshold value relative to normal, when viewedfrom a machining direction. The nesting position in which therestoration design has the lowest negative slope value, corresponding tothe percentage of surface area with negative slope greater than or equalto a specified threshold value for inner surface and/or occlusalsurfaces, is selected as the nesting position from which to calculate atool path.

A flowchart comprising a method (900) for nesting a restoration designis provided in FIG. 9. In one embodiment, a method comprises obtainingpatient's custom restoration design, and mesial and distal contactinformation (901), and determining if a restoration fits within thegeometry of the preform body (902). In one embodiment, a threshold valuemay be established so that the distance from the preform outer surfaceand the restoration design outer surface is greater than a set value,such as greater than 0.2 mm. If the restoration design does not fitwithin the preform geometry, the program may be exited, and a differentrestoration option may be pursued by the dentist If the restorationdesign fits within the preform geometry, nesting options may beidentified having a preform stem location that is outside of mesial anddistal contact areas (904) of adjacent teeth in the same arch. For eachnesting option, the sum of the restoration surface areas (x-y plane)having a negative slope greater than a threshold value is calculated forboth the occlusal and inner surfaces, and then summed. A nesting optionhaving the lowest percentage of surface area having a negative slopevalue (905) that is greater than a threshold value may be selected. Forthe selected nesting option, the restoration design is nested at thebottom of preform adjacent the stem (906). For the selected nestingoption, the stem is positioned coincident to the parting line of therestoration (907), or the line around the position of maximum dimensionbetween the restoration's occlusal surface and margin, for example, whenviewed from the occlusal surface. In a further step (908), the distanceof the first stem end from the restoration design tooth margin isdetermined, for example, to determine if the distance between the stemattachment and the restoration margin will be greater than a thresholdvalue (e.g., greater than 2 mm) upon completion of shaping therestoration. If the distance of the stem (at the point of attachment tothe dental restoration) from the margin is less than the thresholdvalue, the tooth restoration margin may be damaged upon removal of thestem after shaping, so the nesting option may be ignored or refused(910), and a second nesting option may be selected having the nextlowest value for the sum of percentage of surface area with negativeslope. Any of the nesting and analysis step processes may be repeateduntil a final nesting option is selected that meets one or morethreshold values, such as the distance of the stem from mesial anddistal contact areas, distance of the stem from margin and/or occlusalsurface, and/or the percentage of Z-negative direction machining forocclusal and inner restoration design surfaces. After selection of afinal nesting position, a tool path sequence (909) is calculated.

In a further embodiment exemplified by the flowchart of FIG. 10, anothermethod (1000) for nesting a restoration design is provided. The methodcomprises obtaining patient's custom restoration design, and mesial anddistal contact information (1001), and identifying nesting options forthe restoration design within the preform body. Nesting options areidentified in which the stem first end does not connect with therestoration design at mesial and/or distal contact points of therestoration design and adjacent teeth. In one embodiment, two nestingoptions may be provided in which the stem contacts the restorationdesign on the buccal side of the restoration, for example, distal-buccalor mesial-buccal stem positions. For each nesting option, the percent ofthe restoration surface areas (x-y plane) having a negative slopegreater than a threshold value is calculated for the occlusal and/orinner surfaces, and optionally, the occlusal and inner surface valuesare summed (1003). A first nesting option having the lowest percentageof surface area with a negative slope (1004) greater than or equal to athreshold value may be identified. The first identified nesting optionmay be selected, and the position of the restoration design within thepreform may be adjusted so that the stem is coincident to the partingline (1004). For the first selected nesting option, it may be determinedif the restoration design fits within the preform body (1005), forexample, if the distance from the preform outer surface and therestoration design outer surface is greater than a minimum thresholdvalue, such as greater than 0.2 mm. It may also be determined if thedistance between the stem and the margin is greater than a thresholddistance (1006), e.g., 1 mm. If both parameters of steps (1005) and(1006) are met (1007), the first nesting position may be accepted, and atool path may be calculated (1008) from the nesting position of therestoration within the preform. If either parameter of (1005) or (1006)is not met, another nesting option having the next lowest percent ofsurface area with a negative slope may be identified (1010, 1009), andevaluated by the processes of (1004), (1005), and (1006). If a second orsubsequent nesting option meets the parameters of (1005) and (1006), theuser of the program may be notified that a second or subsequent nestingoption meets the parameters, and the user may be prompted for furtheraction, such as to evaluate the nesting option, to select the nestingoption, to provide further adjustment to the nesting option, or toselect another restoration option. If no nesting options meet theparameters of (1005) and/or (1006), the first nesting option having thelowest surface area may be selected (1011), and adjusted, for example,in the Z-axis direction within the preform to establish the bestpossible fit of the restoration design within the preform. The user maybe notified (1012) that no options met all parameters, and the user maybe prompted for further action, as described above.

Nesting software may be separate from the dental restoration designsoftware or may comprise a module of the design software which may beautomatically implemented upon completion of the restoration design.Nesting information comprising positional data of the stem relative tothe dental restoration resulting from the nesting step may be used forcomputing tools paths. A tool path sequence may be calculated from thepositional data of the preform stem and restoration design, which may besplit into two or more tool paths. FIGS. 8A, 8B and 8C illustrate arestoration design (804) nested within a model of a preform (802) inwhich machining steps (800) are split between a front (803) portion anda back (808) portion of the preform body. FIGS. 8A and 8B depict oneembodiment of a machining step for a front portion of the preform body(803), corresponding to a portion of the preform body (802) on the sideopposite the stem (801); FIG. 8C depicts machining step for machiningthe back (808) portion of the preform body, which is adjacent thepreform stem (801). FIG. 8A illustrates a front portion (803) as seenfrom the bottom (809) of the preform body (802) having a cavity (806).In FIG. 8A, the inner surface (805) of the restoration design is nestedadjacent a preform cavity (806) located on the preform bottom (809).FIG. 8B illustrates a front portion (803) as seen from the top (810) ofthe preform, and the occlusal surface (807) of the restoration design isadjacent the top (810) of the preform body. FIG. 8C illustrates amachining step for a back (808) portion of a preform body as seen fromthe preform bottom (809), and the restoration design is nested adjacentthe stem (801) of the preform. The front and back portions may eachcomprise one or more machining steps comprising one or more tool pathsto machine a restoration from the preform. In one embodiment, a firsttool path is provided for machining the front (803) portion of a preformbody and a second tool path is provided for machining the back portion(808) of the preform body adjacent the stem. In one embodiment, a firstparallel lacing tool path is provided for machining the front portionand a second parallel lacing tool path is provided for machining theback portion, and the front and back portions are machined from oppositelacing directions relative to the Y-axis.

In FIGS. 8C, 8D and 8E, dimensional limits of the milling machine areillustrated by machining steps of front (803) and back (808) portions.Tool offset positions (for example, 813 and 814) may be calculated basedon the dimensions of the preform body (802) and stem (801). In oneembodiment as exemplified in FIGS. 8F and 8G, two tool paths areprovided having front and back tool entry positions (811 and 815,respectively), and front and back tool stop positions (812 and 816,respectively) for front (803) and back (808) portions, as illustratedfrom a side view of the preform body (802).

In one embodiment, a first machine step comprises a first tool path formachining the back portion (808) of the preform. A first tool path has aback tool entry point (e.g., 815) near the stem (801) side of thepreform, and comprises a first parallel lacing tool path in they-positive direction. A second tool path for machining the front portionhas a front tool entry point (e.g., 811) at the front side (803) of thepreform, opposite the stem, and comprises a second parallel lacing toolpath in Y-negative direction. The first tool path and second tool pathshaving tool entry points at opposite sides (front and back) of thepreform, each follow a lacing pattern that proceed in oppositedirections (relative to the Y-axis) toward a point on the y-axis wherethe tool paths separate. The tool path sequence may be split into firstand second tool paths at an arbitrary point relative to the y-axis(e.g., FIGS. 8D and 8E), or the tool path sequence may be split intomultiple tool paths based on positional information of the nestedrestoration, selected to optimize machining parameters described herein.In one embodiment, a tool path sequence for machining the inner (orcavity) surface of the restoration may be separated into the first andsecond tool paths (with tool path lines generally parallel x-axisdirection) at the lowest point (e.g., 712, 818) of the restorationdesign tooth margin line on the Y-axis that is closest to the preformstem within a set threshold value. The tool path sequence for machiningthe occlusal surface of the restoration may be separated into first andsecond tool paths (where tool path lines are generally parallel to theX-axis) at a point (e.g., 110, 711, or 817) relative to the Y-axis inwhich the edge of the occlusal surface of the restoration design slopesnegatively toward the stem at a specified angle.

In an embodiment, a method for shaping a preform body into a dentalrestoration comprises, at least two tool paths for shaping the top sideof a preform body comprising i) a first tool path having a tool pathentry at the front end (803) of a preform body (810), and ii) a secondtool path having a tool path entry at the back end of a preform bodyadjacent the stem, and at least two tool paths for shaping the bottomside of a preform body that comprise i) a first tool path having a toolpath entry at the front end of a preform top end (810), and ii) a secondtool path having a tool path entry at the back end of the preform bodyadjacent the stem (801).

In one embodiment, a method for shaping a dental restoration designcomprises two or more tool paths for shaping the occlusal surface andthe inner surface of a restoration design, wherein a tool path forshaping the occlusal surface and a tool path for shaping the innersurface are separated along the restoration parting line that followsthe contour or largest dimension of the outer surface of therestoration. In one embodiment, a machining strategy is providedcomprising i) a first machining step for machining a first portion of anocclusal side of a restoration design adjacent a stem of a preform body,ii) a second machining step for machining a second portion of anocclusal side of a restoration nested adjacent the front portion of thepreform body, iii) a third machining step for machining a first portionof an inner surface side of a restoration design nested adjacent thestem of the preform body, and iv) a forth machining step for machining asecond portion of the inner surface side nested near the front portionof the preform body, wherein each step comprises a separate tool path.In one embodiment, the tool enters the first and third tool pathsadjacent the stem between the preform body and the attachment,contacting the preform body and removing preform material with a toolside surface, thereby reducing the amount of grinding or materialremoval by the tool tip. In an embodiment where the preform bodycomprises a cylindrical form, the tool enters the tool path for thesecond and fourth tool paths (as described above) adjacent the curvedouter surface of the preform on a preform side opposite the stemattachment, wherein the side surface of the grinding tool removespreform material upon entry into the tool path. In one alternateembodiment, either the occlusal surface or the inner surface of arestoration design may be shaped with a single tool path, and the toolpath may have an entry point at the front of the preform body or at theback of the preform body adjacent the stem. CAM software may beimplemented to generate separate tool paths for front and back portionsof the preform body, and the starting and stopping points of the frontand back tool paths are determined by identifying a location to separatethe tool paths relative to the Y-axis. Front and back tool paths of therestoration design may overlap, for example by about 0.2 mm, to provideblending of the tool path lines at the area in which the tool paths aresplit. The front and back tool path sequences for the occlusal surfacemay be split at a location on the Y-axis that is independent of thefront and back tool path separation point for the inner surface.

In a further embodiment, a machining step is provided for reducing thedimension of the stem (width or diameter) at the first stem end tofacilitate removal of the stem from the shaped restoration design uponcompletion of the restoration shaping processes described herein. Afurther tool path may be incorporated into the methods provided hereinthat has a continuous rotary machining path around the stem length axis,reducing stem width or diameter near the point of contact with thedental restoration.

The order of machining steps and tool paths may vary, and terms such asfirst and second, for example, as used in first tool path, second toolpath, third tool path, first machine step, second machine step, and soforth, are used for descriptive convenience, and should not be connotedas indicative of a specific order of steps unless otherwise noted. Analgorithm is provided that comprises parameters that optimize forZ-positive direction positioning of the grinding tool during machiningby identifying a nesting position of the restoration design relative tothe preform stem as determined based on the methods described herein,and, for an identified nesting position, identifying points on theocclusal and inner side surfaces to split the restoration design intoseparate tool paths according to the methods disclosed herein.

In a further embodiment, material feed rates may be individuallycontrolled for each machine step. Machining parameters for front andback tool paths may comprise different material feed rates for front andback portions of the preform body. In one embodiment, a first machinestep for machining a back portion of the preform body having a toolentry point adjacent the stem comprises a first material feed rate thatis faster than the material feed rate of the machining steps formachining the front portion of the preform body when machining theocclusal surface of the restoration design.

Machining parameters may be implemented on a 3+1 axis CNC machine toshape a finished dental restoration from a preform body comprised ofmaterial having a Vickers hardness value greater than or equal to aboutHV4 GPa with a single grinding tool comprising an alloy coating embeddedwith diamonds in a chair-side application. In another embodiment, thecustom dental restoration may be machined in a 3+2, or 4, or 5 axismachine. Corresponding 3+2 or 4 or 5 axis machining cycles may be usedto specify a tool axis angle relative to the tool contact normal of themachined surface either directly from the CAD data of the restoration,or indirectly using a separate tool axis drive surface interpolated fromthe original CAD data. In a further embodiment, more than one grindingtool may be used for grinding the preform. Multiple grinding tools maybe used sequentially, for example, for roughing and finishing, ormultiple grinding tools may be used simultaneously, on opposite surfacesof the preform.

The methods described herein provide enhanced machining in chairsideapplications for preform materials having a Vickers hardness valuegreater than or equal to about HV 4 GPa (Vickers macro-hardness), or avalue in the range of HV 4 GPa to HV 20 GPa (i.e., HV 4 GPa to HV 20GPa), when measured according to the method provided herein.Alternatively, preform materials have a Vickers Hardness value betweenHV 5 GPa and HV 15 GPa, or between HV 11 GPa and HV 14 GPa. Preform bodymaterials comprising hardness values within this range may includemetals, such as cobalt chrome, glass and glass ceramics, such as lithiumsilicate and lithium disilicate, and ceramic materials, includingsintered ceramics comprising alumina and zirconia.

Dental restoration materials, including but not limited to commerciallyavailable dental glass, glass ceramic or ceramic, or combinationsthereof, may be used for making the preforms that are machinable by themethods described herein. Ceramic materials may comprise zirconia,alumina, yttria, hafnium oxide, tantalum oxide, titanium oxide, niobiumoxide and mixtures thereof. Zirconia ceramic materials include materialscomprised predominantly of zirconia, including those materials in whichzirconia is present in an amount of about 85% to about 100% weightpercent of the ceramic material. Zirconia ceramics may comprisezirconia, stabilized zirconia, such as tetragonal, stabilized zirconia,and mixtures thereof. Yttria-stabilized zirconia may comprise about 3mol % to about 6 mol % yttria-stabilized zirconia, or about 2 mol % toabout 7 mol % yttria-stabilized zirconia. Examples of stabilizedzirconia suitable for use herein include, but are not limited to,yttria-stabilized zirconia commercially available from (for example,through Tosoh USA, as TZ-3Y grades). Methods form making dental ceramicsalso suitable for use herein may be found in commonly owned U.S. Pat.No. 8,298,329, which is hereby incorporated herein in its entirety.

The preform body may be made from unsintered materials shaped into anintermediate form having substantially the same geometry as the sinteredpreform, but with enlarged dimensions to accommodate shrinkage uponsintering, where necessary. Suitable unsintered ceramic materials may bemade into blocks by processes including molding and pressing, includingbiaxial or iso-static pressing, and may optionally comprise binders andprocessing aids. Ceramic blocks may be shaded so that the sinteredpreforms have the color of natural or artificial dentition, requiring nofurther coloring after formation of the dental restoration. Coloringagents may be incorporated during block formation to more closely matchthe appearance of natural or commercially available artificial dentitionthan uncolored or unshaded ceramic materials. Optionally, ceramic powdermay be processed into blocks by slip casting processes, includingprocesses described in commonly owned U.S. Patent Publication Nos.2009/0115084; 2013/0231239; and 2013/0313738, incorporated by referencein their entirety. Pre-sintered ceramic blocks suitable for use inmaking intermediate shaped forms include commercially available ceramicmilling blocks including those sold under the trade name BruxZir® (forexample, BruxZir® Shaded 16 Milling Blanks, Glidewell Direct, Irvine,Calif.). In some embodiments, the theoretical maximum density of fullysintered zirconia ceramics is between about 5.9 g/cm³ to about 6.1g/cm³, or for example, or about 6.08 g/cm³

A unitary preform may be shaped from a from a single continuousgreen-state block or pre-sintered ceramic block, requiring no separateattachment step for attaching the stem and/or attaching member to thepreform body. Alternatively, the preform may be made by known moldingprocesses, including injection molding. The intermediate shaped form maybe sintered to a density greater than about 95% of the theoreticalmaximum density by known sintering protocols. Sintered zirconia ceramicpreforms may have densities greater than about 95%, or greater thanabout 98% or greater than about 99%, or greater than about 99.5%, of themaximum theoretical density of the ceramic body.

The preform body comprises materials that are shapeable into dentalrestorations in chair-side applications by the methods described herein,that have acceptable strength properties for use in anterior, posterioror both anterior and posterior dental restoration applications, withoutadditional post-shaping processing steps to alter the material strengthproperties after shaping, such as by sintering. Sintered preforms maycomprise zirconia ceramic materials that have high flexural strength,including strength values greater than about 400 MPa, or greater thanabout 500 MPa, or greater than about 600 MPa, or greater than about 800MPA, when tested by a flexural strength test method for zirconiamaterials as outlined in ISO 6872:2008, as measured and calculatedaccording to the 3 point flexural strength test described forDentistry—Ceramic Materials.

Dental restorations may be made by grinding sintered ceramic preformsusing grinding tools instead of traditional milling tools because of thematerial hardness which renders typical milling tools unsuitable incertain embodiments. Grinding tools having a diamond coating, includingnickel plated tools embedded with diamonds, are suitable for use herein.A grinding tool (300) having a shank (301), for example as illustratedin FIG. 3, comprises an embedded diamond coating on the shank (301) andtip. Diamonds suitable for use herein include blocky or friable diamondshaving an average size in the range of about 90 micron to about 250micron, or an average size in the range of about 107 micron to about 250micron, or an average size in the range of about 120 micron to about 250micron, or for example, an average size in the range of about 120 micronto about 180 micron. Suitable diamond coatings include those in which atleast 50% of the diamonds are embedded by a metal alloy layer for morethan half the height of the diamond, for example, as determined by SEManalysis. Grinding tools having a coating in which diamonds are embeddedin a metal alloy to a depth of about 50% to 95% of the diamond height,or about 60% to about 95% of the diamond height, or to about 80% toabout 95% of the diamond height are useful for shaping preforms madefrom materials such as fully sintered zirconia preforms, or preformscomprising materials having the hardness values described herein. Insome embodiments, grinding tools have a diamond coated shank with ametal alloy layer having thickness that is greater than about 50% of thediamond grit size (e.g., in microns), or greater than about 60%, orgreater than about 70%, or greater than about 80%, or greater than 90%,or between about 60% and 90%, or between about 80% and 100%, of thediamond grit size (e.g., in microns). In one embodiment, a grinding toolhas a diamond coated shank comprising a diamond size in the range of 126grit to 181 grit, and a nickel alloy layer having a thickness that isgreater than or equal to about 70% of the diamond grit size (inmicrons).

Test Methods

Flexural Strength Test

Flexure tests were performed on sintered test materials using theInstron-Flexural Strength test method for zirconia materials as outlinedin ISO 6872:2008.

Test bars were prepared by cutting bisque materials taking intoconsideration the targeted dimensions of the sintered test bars and theenlargement factor (E.F.) of the material, as follows:

starting thickness=3 mm×E.F.;

starting width=4 mm×E.F.;

starting length=55 mm×E.F.

The cut, bisque bars were sintered substantially according to thesintering profile provided by manufacturer of the bisque material.Flexural strength data was measured and calculated according to the 3point flexural strength test described in ISO (International Standard)6872 Dentistry—Ceramic Materials.

Vickers Hardness Number

Preform materials may be tested for hardness using a Vickers hardness(macro-hardness test). Hardness numbers (HV) may be calculated asdescribed in ISO-6507, or by determining the ratio of F/A where F is theforce applied in kg/m² and A is the surface area of the resultingindentation (mm²). HV numbers may be converted to SI units and reportedin units, HV GPa, as follows: H(GPa)=0.009817HV.

Examples 1-21

Twenty-one zirconia crowns of multiple tooth types (numbers) were shapedfrom sintered zirconia preforms by the methods described herein.

Partially sintered zirconia milling blocks were obtained (BruxZir®Shaded milling blocks, Glidewell Direct, Irvine, Calif.) and milled intothe shape of a preform by standard milling procedures incorporating anenlargement factor calculated from the block density. The pre-sintered,unitary shaped forms had a cylindrical body, stem and attaching membersubstantially as depicted in FIGS. 2B-2D, and having a cavity extendinginwardly from a bottom surface. The stem had sufficient length betweenthe attaching member and the cylindrical body after sintering forpositioning the tip of a ball nose grinding tool in the z-axis directionwithout contacting the sintered preform. The attaching member shape andsize was compatible for attaching to a mandrel used with the CNC machinein the grinding process.

The pre-sintered shaped forms were sintered according to sinteringprofile of the zirconia blocks provided by the manufacturer to formfully sintered zirconia preforms having a density between about 5.9g/cm³ and 6.1 g/c m³. The fully sintered preforms had a body lengthbetween about 12.8 mm and 14.2 mm, a cross-sectional outer diameter ofabout 14 mm to 15 mm, a cavity breakout diameter on the bottom endsurface having a diameter of about 7 mm to 8 mm; the cavity contour wasconical having a depth of about 4 mm. A first stem end had a width ofabout 2-2.8 and the stem length was between about 6.8 and 7.3 mm.

The preforms each comprised an attachment having a bottom surface gluedto metallic mandrels. The preforms were shaped into finishedrestorations of multiple tooth shapes based on CAD design files using a3+1 axis CNC machine (TS150™ Chairside mill system, IOS Technologies,San Diego, Calif.) having z-, x-, and y-axes directional movement ofgrinding tool, plus rotation of the preform between tool path cycles).The grinding tool comprised diamonds (size: about 126 micron) embeddedin a nickel alloy plating to an embedded diamond depth of about 80%-90%.A CAM lacing cycle, with step over capability in both planar and thegrinding tool axial direction, was utilized to grind all surfaces of thecrown. The grinding tool had an average diamond grit size between about90-210.

An air spindle with rotational speed of about 150000 rpm and a minimuminlet air pressure of about 85 psi were used to grind the fully sinteredzirconia preform. The CAM lacing cycle parameters were determined basedon tooth number, and grinding surface of the sintered preform relativeto the top or bottom of the restoration, the stem side or the sideopposite the stem side of the restoration). Restorations were made fromthe sintered preforms for posterior teeth numbers 2, 3, 14, 15, 18, 19,30 and 31 in under 60 minutes as seen in Table 1.

TABLE 1 Time to completed restoration from Sintered Preform in MinutesTime (Minutes) for each Example, Example Number Tooth # respectively 1;2; 3; 4 2 52; 47; 51; 44 5; 6; 7; 8 3 53; 49; 54; 57 9; 10 14 47; 51 1115 38 12; 13; 14 18 54; 55; 47 15 19 49 16; 17; 18; 19 30 53; 48; 50; 5020; 21 31 49; 50

FIG. 1 illustrates a restoration crown made according to the Examplesdescribed in Table 1. Restoration crowns were shaped having minimalresidual material (106) remaining between the stem and restoration aftergrinding is completed. The restoration crown was snapped off of thestem, and the location of the stem was hand-sanded to a smooth finish,where necessary.

Examples 22-44

Twenty-two zirconia crowns representing six restoration designs ofmultiple tooth types (Tooth #2, 14, 15, 30, and 31) were shaped fromsintered zirconia preforms that were made substantially according to themethods provided in Examples 1-21.

Restoration designs were generated for multiple tooth preparationscorresponding to the following tooth numbers: #2—second molar on upperarch; #14 and #15—first and second molars, respectively, on an upperarch; #30 and #31—first and second molars, respectively, on lower archusing a dental CAD system (IOS™ FASTDESIGN) based on patient scan data.The six custom dental restoration designs were nested within a computermodel of a preform to provide placement of the preform stem relative tothe tooth design, according to Table 2, with reference to FIGS. 6A and6B.

Each restoration design was nested four times to provide four nesteddesigns. In each nesting operation, the stem of the preform waspositioned in one of four different locations relative to therestoration, as indicated in Table 2. For example, for restorationdesigns nested with a Number 1 stem position, the Y-axis of the stem waspositioned between mesial contact area and buccal surfaces of therestoration design. In the Number 2 stem position, the stem was locatedbetween distal contact area and buccal surfaces. In the Number 3 stemposition, the stem was located between distal contact area and lingualsurface, and in the Number 4 stem position, the stem was located betweenmesial contact area and lingual surface.

TABLE 2 Machining Time For Shaping Dental Restorations From SinteredPreforms Time (in Stem Successful minutes) to Example No. Tooth #Position No. Stem Location Completion Completion 22 31 1 Mesio-buccalYes 49 23 2 Disto-buccal Yes 53 24 3 Disto-lingual Yes 50 25 4Mesio-lingual Yes 52 26 14 1 Mesio-buccal Yes 47 27 2 Disto-buccal Yes49 28 3 Disto-lingual Yes 48 29 4 Mesio-lingual No — 30 14 1Mesio-buccal Yes 51 31 2 Disto-buccal Yes 50 32 3 Disto-lingual Yes 5433 4 Mesio-lingual No — 34 30 1 Mesio-buccal Yes 50 35 2 Disto-buccalYes 52 36 3 Disto-lingual Yes 59 37 4 Mesio-lingual Yes 59 38 2 1Disto-buccal Yes 54 39 2 Mesio-buccal Yes 49 40 3 Mesio-lingual Yes 5241 4 Disto-lingual No — 42 15 1 Mesio-buccal Yes 38 43 2 Disto-buccal No— 44 3 Disto-lingual Yes 42 45 4 Mesio-lingual No —

Positional nesting information was used to calculate four tool pathsequences for restoration designs, wherein each restoration designcomprised four tool paths. Two tool paths were generated for machiningthe front portion of the preform and back portion (adjacent the stem) ofthe preform body for each surface (the occlusal surface and innersurface (cavity side)) of the restoration designs. Restoration designswere machined for each preform using the calculated tool paths.

The results of each example were assessed. Success or failure of shapinga completed restoration is indicated in Table 2, where a lack of success(“No”) occurred from tool damage prior to completion of the restoration.The time to shape a restoration from the sintered preform based onnesting conditions was also calculated as seen in Table 2.

All restoration designs in which the tool paths were calculated fromnesting operations in which the stem was positioned in a mesio-buccalposition were successfully shaped in less than one hour. Eleven out oftwelve restoration designs were successfully shaped in under 1 hour fromtool paths calculated from nesting positions in which the stems were ina mesio-buccal or disto-buccal position. Three out of 5 restorationdesigns in which the tool paths were calculated based on positional datain which the stem was nested in the mesio-lingual position wereunsuccessfully shaped. The shortest shaping time to shape all successfulrestorations occurred for tool paths calculated from nesting positionsin which the stem location was buccally oriented (a mesio-buccal ordisto-buccal position).

We claim:
 1. A method for making a dental restoration from a sinteredceramic preform, comprising: obtaining a 3D CAD file of a dentalrestoration design and a computer model of a preform, wherein thepreform comprises a preform body and a preform stem that projects froman outer surface of the preform body at a stem contact point; nestingthe dental restoration design within the computer model of the preformbody by identifying at least two nesting options, wherein the locationof the stem contact point relative to the dental restoration design isdifferent for the at least two nesting options, and selecting one of theat least two nesting options; generating machining instructions forshaping the dental restoration from the preform based on the selectednesting option; and machining a sintered ceramic preform body into afinal, posterior dental restoration crown with a single grinding tool inless than 1 hour of machining time.
 2. The method of claim 1, whereinthe step of nesting comprises selecting a nesting option in which thestem contact point is outside of a mesial contact area and a distalcontact area of the dental restoration design.
 3. The method of claim 1,wherein the step of nesting comprises selecting a nesting option inwhich the stem contact point is adjacent a mesio-buccal position of therestoration design.
 4. The method of claim 1, wherein the step ofnesting comprises selecting a nesting option in which the stem contactpoint is adjacent a disto-buccal position of the restoration design. 5.The method of claim 1, further comprising a step of calculating anegative slope value for an occlusal surface, an inner surface, or bothocclusal and inner surfaces, of the restoration design for at least twonesting options, and selecting the nesting option having the lowestnegative slope value.
 6. The method of claim 1, wherein the preform bodycomprises a cavity and the dental restoration design comprises an innersurface, and the step of nesting further comprises nesting a preformbody cavity adjacent the inner surface of the restoration design.
 7. Themethod of claim 1 comprising generating a first machining step having afirst tool path entry position adjacent the stem.
 8. The method of claim1 comprising at least two machining steps for machining an occlusal sideof a dental restoration and at least two machining steps for machiningan inner surface side of a dental restoration.
 9. The method of claim 8,comprising generating a tool path for the occlusal side and a tool pathfor the inner surface side and the tool paths are separated from eachother at a dental restoration parting line.
 10. The method of claim 8wherein the step of generating machining instructions comprisesgenerating i) a first tool path having a tool path entry at a front endof the preform body for shaping a first portion of the occlusal side ofa dental restoration; ii) a second tool path having a tool path entry ata back end of the preform body adjacent the stem for shaping a secondportion of the occlusal side of a dental restoration; iii) a third toolpath having a tool path entry at the front end of the preform body forshaping a first portion of the inner surface side of a dentalrestoration; and iv) a fourth tool path having a tool path entry at theback end of the preform body adjacent the stem for shaping a secondportion of the inner surface side of a dental restoration.
 11. Themethod of claim 1, wherein the at least one machining step comprises atool path comprising adjacent tool path lines, and the method furthercomprises inserting additional tool path lines between two adjacent toolpath lines if a Z increment between the adjacent tool path lines exceedsa maximum threshold value in a Z-negative machining direction.
 12. Themethod of claim 1 wherein the sintered ceramic preform body comprises amaterial having a hardness value greater than or equal to about HV 4 GPa(macro-hardness).
 13. The method of claim 1 wherein the sintered ceramicpreform body material is a fully sintered ceramic material thatcomprises greater than 85% zirconia.
 14. The method of claim 1 whereinthe preform body comprises a cylindrical shape having top and bottomsurfaces, a circular cross-section, and a curved outer surface thatextends between top and bottom surfaces, and the stem extends outwardlyfrom the stem contact point on the curved outer surface.
 15. The methodof claim 1, wherein the method comprises machining the sintered ceramicpreform body with a grinding tool that comprises a diamond coatingwherein the diamonds are embedded in a metal alloy layer.
 16. The methodof claim 15, wherein the grinding tool comprises diamonds embedded in ametal alloy layer having a thickness that is approximately greater thanor equal to 80% the diamond grit size (in microns).
 17. The method ofclaim 15, wherein the at least one machining step comprises a tool pathcomprising adjacent tool path lines, and a threshold value for a maximumZ increment value in a Z-negative direction between the adjacent toolpath lines that is approximately equal to a distance that is less thanor equal to approximately 10% of the diamond grit size.
 18. The methodof claim 1, wherein the dental restoration is a dental crown.
 19. Amethod for making a dental restoration from a preform comprising fullysintered ceramic material shaded to achieve a color of natural dentitionin a final dental restoration comprising: obtaining a 3D CAD file of adental restoration design and a computer model of a preform, wherein thepreform comprises a preform body comprising a cylindrical shape and apreform stem that projects from a curved outer surface of the preformbody at a stem contact point, nesting the dental restoration designwithin the computer model of the preform body by identifying a firstnesting option wherein the stem contact point is at a mesio-buccalposition relative to the dental restoration design, and a second nestingoption wherein the stem contact point is at a disto-buccal positionrelative to the dental restoration design, and selecting one of the twonesting options; generating machining instructions for shaping thedental restoration from the preform based on the selected nestingoption; and machining a sintered ceramic preform body into a final,posterior dental restoration crown with a single grinding tool in lessthan 1 hour of machining time.